If it is a one time blip, I wouldn't be concerned. If it becomes a pattern of the numbers staying lower and not recovering or constantly bouncing around, then I'd worry more. For instance, my one dd who is pretty consistent in her test results has had a few instances where we've seen scores drop like this. It is usually related to anxiety or just a bad day. It is also something that, the next time she tests, goes away (i.e. the next set of test scores is back in line with the prior scores or higher). My other kiddo is truly erratic and may see major drops that take years to return to places they were before or which vary radically. She is 2e which likely plays into it.
The only time I've seen something in the case of my more consistent one that gave me pause regarding her education was with her math MAPS scores in 4th grade. It reflected what I saw in her learning experience that year: she progressed almost not at all and her scores were nearly identical from the fall to spring. She had also been taught virtually nothing in math that year so it felt like a legitimate stagnation.
Given that the 3rd grade learning experience was poor for your dd, I think that it is valid to approach her teacher with a discussion about what can be done to reignite the fire re reading, but I'd suspect that the SRI lexile score this time doesn't reflect a true regression. We saw a loss of love of reading for my oldest in 1st grade with a truly awful teacher. For her, all it took to get it back was a lot of reading time with me with good books. I homeschooled her for the end of that school year so it was easy to find the time but we essentially read the Harry Potter series together: she read a few pages then I read a few pages. Your dd is older so that may not be the series for her (or she may have already read it), but if you can find something in a genre she loves and maybe spend some time reading together, perhaps it will help (?).
I'm sorry that she's had a bad experience like that. It is crummy to see a child who once loved to read have that extinguished. FWIW, mine loves reading again but still bears a grudge against her 1st grade teacher and she's a 10th grader now.
